The demand for dot matrix display devices typified by liquid crystal display devices has been rapidly increasing for portable applications as well as fixed applications such as a TV receiver and a display for a personal computer. In recent years, as a next-generation display device replacing liquid crystal display devices, EL display devices that have pixels each including an organic electroluminescence (EL) element have been put into practical use.
A dot matrix display device is generally classified into a passive matrix display device and an active matrix display device. In the active matrix display device, a gray scale is displayed by an analog gray scale method or a digital gray scale method. The analog gray scale method is a method of displaying a gray scale by controlling the luminance of pixels. In the digital gray scale method, each pixel is controlled by two values: whether light is emitted or not. A gray scale is displayed by the size of a light emitting area or the length of a light emitting time in a certain period, and the former is called an area gray scale method while the latter is called a time gray scale method.
In the aforementioned time gray scale method, one frame period is divided into a plurality of subframe periods, and the light emitting time in each subframe period is set different. Then, the luminance in one frame period is controlled by combining the subframe periods to display a gray scale. One of the methods of achieving a multi-gray scale in this manner is disclosed in Patent Document 1.
According to Patent Document 1, in the case of, for example, a 6-bit display (64 gray scale levels), one frame period is divided into six subframe periods (SF1 to SF6), the length of a light emitting time in each subframe period is set 25:24:23:22:21:1, and each gray scale level is displayed by selecting the subframe period where light is emitted (see FIG. 5A). Specifically, if light is emitted in none of the subframe periods, the first gray scale level (black: luminance 0) is displayed, and if light is emitted in all of the subframe periods, the 64th gray scale level (white: luminance 63) is displayed. Meanwhile, if the light emitting times having lengths of 24, 23, 22, and 1 are selected, 24+23+22+1=29, namely the 30th gray scale level (luminance 29) is displayed among the 64 gray scale levels from the luminance 0 to the luminance 63.
In a lower bit, namely in a subframe period having a short light emitting time, it is necessary to stop light emission before the next subframe period starts. Thus, one-row selection period is divided into a plurality of sub-horizontal periods (see FIG. 5B, where one-row selection period is divided into two sub-horizontal periods), and a video signal is written in one sub-horizontal period while a video signal is erased in another sub-horizontal period. The light emitting time of each bit is controlled by performing the writing operation and the erasing operation in an appropriate row at an appropriate timing. The writing operation and the erasing operation are performed by the corresponding gate drivers (also called gate signal line driver circuits).    [Patent Document 1] Japanese Patent Laid-Open No. 2001-324958